1. Technical Field
This disclosure relates to power converters and, more particularly, to DC-to-DC converters that use pulse frequency modulation (“PFM”).
2. Description of Related Art
Power converters typically convert power from one level to another, while conserving power in the transformation. For example, a supply voltage may be converted from one voltage to another or to a current.
One type of power converter is a DC-to-DC converter. These are often referred to as point of load converters where the input voltage is converted to a desired output voltage from an input source at the point of load. Such a converter may convert a direct current (“DC”) voltage at one level to a DC voltage at a different level, such as to a lower level. To accomplish this, an electronic switch and an inductor may be connected in series between a DC source voltage supply and a load (which may include filtering capacitance). The electronic switch may be configured to control the supply of current through the inductor in response to a control signal that is delivered to the electronic switch. The control signal may be a series of pulses. The electronic switch may conduct current from the source voltage supply to the inductor at such times as the pulses are high. By regulating the pulses, the energy which is delivered into the inductor may be regulated and, in turn, the voltage that is delivered to the load.
A pulse controller circuit may be used to generate the pulses that are delivered to the electronic switch. The pulse controller circuit may be configured to generate pulses having a fixed pulse width, commonly referred to as a constant on time. The amount of energy that is delivered into the inductor may thus be regulated by controlling the frequency of these constant-width pulses. Typically, a feedback loop is provided which causes the frequency of the pulses to increase in response to increases in the load on the DC-to-DC converter.
Changes in the load on the converter may require corresponding changes in the amount of energy which is delivered into the inductor in order to maintain the output load voltage constant. In turn, this may require changes in the frequency of the pulses that are delivered to the electronic switch.
Unfortunately, conditions can arise when constant on-time, pulse controller circuits fail to respond quickly enough to changes in the load on the converter, thus causing the output voltage to deviate from a target value for an undesirably long period. For example, a rapid increase in the load on the converter, commonly referred to as a “load step,” may cause a corresponding rapid decrease in the output voltage. In response, the pulse controller circuit may increase the frequency of the pulses to increase the energy that is delivered to the load so as to bring the voltage back to the target value. However, the maximum pulse frequency which can be produced by these pulse controller circuits may be limited by a minimum off time (TOFF(min)) between each pulse. This may limit the ability of these pulse controller circuits to quickly bring the load voltage back to the target value in response to the load step.
A similar delay may result in connection with a rapid decrease in the load on the converter, commonly referred to as a “load release.” A load release may cause an undesirable jump in the load voltage. When a load release occurs immediately after the pulse controller circuit initiates a constant on-time pulse, energy may continue to be delivered into the inductor, even though the load has been decreased. This continued delivery of energy into the inductor may cause the load voltage to increase even further, when exactly the opposite may be needed. This may again reduce the ability of the pulse controller circuit to rapidly bring the load voltage back to the target value in response to a transient condition.
In short, constant width PFM DC-to-DC-converters may fail to recover in response to transients caused by load steps and/or load releases as quickly as may be desired.